1. Field.
This invention pertains to the field of measurements, and more particularly, to a geometric measurement system and a method of making geometric measurements of an object using light reflected and/or refracted from one or more surfaces of the object.
2. Description.
There are many examples of measurement or metrology systems that are designed to measure or characterize an object's surface. Among these systems are optically-based systems which operate by reflecting or scattering light from the object's surface and then collecting and analyzing the reflected or scattered light. These systems may use any of a number of principles such as, but not limited to, interferometry, Moiré´deflectometry, heterodyne interferometry, laser triangulation, phase diversity wavefront sensing, or Shack-Hartmann (Hartmann-Shack) wavefront sensing. Accurate measurements are possible with some of these techniques down to a fraction of a nanometer.
However, many of these techniques are difficult to apply to highly curved surfaces and/or optically transmissive surfaces. When objects such as, but not limited to, a contact lens, contact lens mold, high numerical aperture optical element, pin, optical lens, inter-ocular lens (IOL), IOL mold, curved mirror, cornea, or another object with a rapid variation in surface contour is measured, it is very difficult to project the light onto the entire surface and to collect it back in a controlled and uniform fashion. Projecting and collecting lens(es) with a very high numerical aperture (NA) are required. Furthermore, while good results may be achieved with some of these methods by projecting and then collecting the light from a spherical surface, the degree to which the surface can depart from spherical is limited by the dynamic range of the measurement instrument. This severely limits the range of objects whose surfaces can be measured since many objects are not spherical but may be highly aspherical.
Also, many objects are optically transmissive at the wavelength of the light used in the above-mentioned measuring systems. In that case, in projecting and collecting light from the object, light is collected from all surfaces simultaneously. The light reflected from various surfaces mixes together and makes interpretation of resulting patterns difficult. The different surfaces may reflect vastly different amounts of light depending upon the index of refraction and other conditions of the surfaces. While it is possible in some cases to spoil the reflection (or otherwise identify it) from the back surface (or other feature that is not of interest) by painting it black, immersing it in a fluid, or otherwise altering it, this has the effect of damaging the part that is being measured. This is not generally desirable in a measurement system.
It is also possible to use a contact profilometer to measure the surface. Very sophisticated versions of these instruments exist and they are capable of making very precise measurements. However, it is generally not possible to measure two surfaces simultaneously with a profilometer, and the fact that there is a contact with the surface may damage the object. In addition, these instruments are very slow and may have different precision on rough surfaces than they do for smooth surfaces.
Neal et al. U.S. Pat. No. 6,184,974 (“Neal et al.”), which is incorporated herein by reference in its entirety as if fully set forth herein, discloses a means for making measurements of a small area of at least one surface of a silicon wafer or other flat surface and for stitching these measurements together to form a measurement of the entire surface. Neal et al. uses overlap regions to connect the measurements together and eliminate any effects of instrument inaccuracy during the measurement process. This same technique has been applied to the measurement of large telescope mirrors with excellent success (Kiikka et al, “The JWST Infrared Scanning Shack Hartmann System: a new in-process way to measure large mirrors during optical fabrication at Tinsley,” SPIE 2006).
It would be desirable to provide a method and system for measuring one or more geometric characteristics of an object having one or more highly curved, potentially aspheric and non-symmetric surfaces. It would also be desirable to provide a method and system for measuring one or more geometric characteristics of an object that has at least one substantially transparent surface, which can accurately distinguish between first and second surface reflections and provide accurate surface shape maps for each surface.
In one aspect of the invention, a method determines at least one geometric characteristic of an object having a first surface and a second surface. The method comprises: (a) adjusting a positional relationship between a first surface of the object and a light source to illuminate a subregion of the first surface of the object, whereby a portion of light illuminating the subregion of the first surface of the object passes through the object to the second surface of the object; (b) delivering light from the subregion of the first surface of the object to a wavefront sensor while blocking a majority of light from the second surface of the object from reaching the wavefront sensor; (c) determining a wavefront of light received from the subregion of the first surface with a wavefront sensor; (d) repeating steps (a) through (c) for a plurality of different subregions spanning a measurement region for the first surface of the object, where adjacent subregions have an overlapping portion; (e) stitching together the wavefronts determined in each execution of step (c) including derivatives of the wavefronts in the overlapping portions, to construct a wavefront of light received from the measurement region of the first surface of the object; and (f) determining at least one shape parameter of the first surface of the object from the constructed wavefront.
In another aspect of the invention, a system determines at least one geometric characteristic of an object. The system comprises: a light source; a wavefront sensor; an optical system adapted to deliver light from the light source to a surface to be measured of the object, and to deliver light from the surface to be measured of the object to the wavefront sensor, whereby a portion of the light delivered to the surface to be measured passes through the object to a surface of the object that is not being measured; a positioner adapted to adjust relative positions of the light source and the surface to be measured such that, at each relative position, the light from the light source is delivered onto a sub-region of the surface to be measured, and light from the sub-region of the surface to be measured is delivered to the wavefront sensor, the positioner adjusting the relative positions such that adjacent sub-regions have an overlap portion; and a processor adapted to stitch together wavefronts measured by the wavefront sensor for different sub-regions of the surface to be measured at the relative positions provided by the positioner, including using derivatives of wavefronts in overlap regions, to construct a wavefront of light received from a measurement region of the surface to be measured, wherein the optical system comprises an aperture for blocking a majority of light from the surface of the object not being measured from reaching the wavefront sensor.
In yet another aspect of the invention, a method determines at least one geometric characteristic of an object having a first surface and a second surface. The method comprises: (a) adjusting a positional relationship between a first surface of the object and a light source to illuminate a subregion of the first surface of the object, including at least one of: rotating the object with respect to the light source, rotating the light source with respect to the object, tilting the object with respect to the light source, and tilting the light source with respect to the object; (b) delivering light from the subregion of the first surface of the object to a wavefront sensor; (c) determining a wavefront of light received from the subregion of the first surface with a wavefront sensor; (d) repeating steps (a) through (c) for a plurality of different subregions spanning a measurement region for the first surface of the object, where adjacent subregions have an overlapping portion; (e) stitching together the wavefronts determined in each execution of step (c) including derivatives of the wavefronts in the overlapping portions, to construct a wavefront of light received from the measurement region of the first surface of the object; and (f) determining at least one shape parameter of the first surface of the object from the constructed wavefront.
In still another aspect of the invention, a system determines at least one geometric characteristic of an object, the system comprising: a light source; a wavefront sensor; an optical system adapted to deliver light from the light source to a surface to be measured of the object, and for delivering light from the surface to be measured of the object to the wavefront sensor; a positioner adapted to adjust relative positions of the light source and the surface to be measured such that, at each relative position, the light from the light source is delivered onto a sub-region of the surface to be measured, and light from the sub-region of the surface to be measured is delivered to the wavefront sensor, the positioner adjusting the relative positions such that adjacent sub-regions have an overlap portion, wherein the positioner includes one of: means for rotating the light source, means for rotating the object, means for tilting the light source, and means for tilting the object; and a processor adapted to stitch together wavefronts measured by the wavefront sensor for different sub-regions of the surface to be measured at the relative positions provided by the positioner, including using derivatives of wavefronts in overlap regions, to construct a wavefront of light received from a measurement region of the surface to be measured.
In a further aspect of the invention, a system determines at least one geometric characteristic of an object having a first surface and a second surface. The system comprises: a light source adapted to illuminate the object; an optical element adapted to receive light from the first and second surfaces of the object and to produce a first light beam corresponding to light from the first surface and a second light beam corresponding to light from the second surface; a light intensity detector having a radiation sensitive surface adapted to receive the first and second light beams and to detect the intensity of incident radiation on the radiation sensitive surface from the first and second light beams, and to produce an output that provides a measure of the intensity of the incident radiation; a positioner adapted to adjust relative positions of the optical element and the light intensity detector; and a processor adapted to determine wavefronts of the light from the first and second surfaces based on the output of the light intensity detector at a plurality of different relative positions.
In a still further aspect of the invention, a method determines at least one geometric characteristic of an object having a first surface and a second surface. The method comprises: (a) illuminating the object; (b) transmitting light from the first and second surfaces of the object through an optical element to produce a first light beam corresponding to light from the first surface and a second light beam corresponding to light from the second surface; (c) detecting the intensity of incident radiation on a radiation sensitive surface from the first and second light beams; (d) adjusting relative positions of the optical element and the radiation sensitive surface, and at each of a plurality of different relative positions producing an output that provides a measure of the intensity of the incident radiation; and (e) determining wavefronts of the light from the first and second surfaces of the object based on the outputs produced at each of the different relative positions.
In yet a further aspect of the invention, a system determines at least one geometric characteristic of an object having a first surface and a second surface. The system comprises: a light source adapted to illuminate the object; a diffractive optical element adapted to receive light from the first and second surfaces of the object and to produce therefrom at least two spatially-separated light distributions having at least one statistical characteristic different from each other; a light intensity detector having a radiation sensitive surface adapted to receive the at least two spatially-separated light distributions, to detect the at least two spatially-separated light distributions at different points across the radiation sensitive surface and to produce an output that provides a measure of the intensity of the incident radiation at the different points; and a processor adapted to determine wavefronts of the light from the first and second surfaces based on the output of the light intensity detector.
In still yet another aspect of the invention, a method determines at least one geometric characteristic of an object having a first surface and a second surface. The method comprises: (a) illuminating the object; (b) transmitting light from the first and second surfaces of the object through a diffractive optical element to produce therefrom at least two spatially-separated light distributions having at least one statistical characteristic different from each other; (c) detecting the intensity of incident radiation on a radiation sensitive surface from the at least two spatially-separated light distributions at different points on the radiation sensitive surface to produce an output that provides a measure of the intensity of the incident radiation at the different points; (d) determining wavefronts of the light from the first and second surfaces of the object based on the output of the detection.
In still yet a further aspect of the invention, a system determines at least one geometric characteristic of an object. The system comprises: a structure having a reference surface with a known curvature; a stage adapted to hold an object; and an interferometer. The interferometer comprises: a light source adapted to generate light having a broad spectral bandwidth, a detector; a mirror; a beamsplitter adapted to receive the light from the light source and to divide the light into a first portion and a second portion; wherein the system is configured to provide the first portion of the light from the beamsplitter to illuminate the object and the reference surface, and to provide at least some of the first portion of the light from the object and the reference surface to the detector, and wherein the system is configured to provide the second portion of the light from the beamsplitter to illuminate the mirror, and to provide at least some of the second portion of the light from the mirror to the detector; and means for adjusting an optical path length traveled by the second portion of the light from the beamsplitter to the detector, wherein the detector is adapted to output a signal indicating when an optical path length traveled by the first portion of the light from the beam splitter to the detector is the same as the optical path length traveled by the second portion of the light from the beamsplitter to the detector.
In another, further aspect of the invention, a method determines at least one geometric characteristic of an object. The method comprises: (a) generating light having a broad spectral bandwidth; (b) dividing the light into a first portion and a second portion with a beamsplitter; (c) providing the first portion of the light to a selected region of the object and to a reference surface of a structure having a known curvature; (d) providing at least some of the first portion of the light from the object and the reference surface to a detector; (e) providing the second portion of the light to a mirror; (f) reflecting the first portion of the light from the mirror to the detector; (g) passing a second portion of the light through a reference lens to a selected region of a surface to be measured of the object; (h) adjusting an optical path length traveled by the second portion of the light from the beamsplitter to the detector until the detector outputs a signal indicating a first interference fringe caused by light refracted or reflected by a first surface of the object; (i) adjusting the optical path length traveled by the second portion of the light from the beamsplitter to the detector until the detector outputs a second signal indicating a second interference fringe caused by light refracted or reflected by a second surface of the object; (j) adjusting the optical path length traveled by the second portion of the light from the beamsplitter to the detector until the detector outputs a second signal indicating a third interference fringe caused by light refracted or reflected by the reference surface; and (k) determining a thickness of the object at the selected region from the first, second, and third interference fringes.